Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21740204s&link_type=abstract
American Astronomical Society, AAS Meeting #217, #402.04; Bulletin of the American Astronomical Society, Vol. 43, 2011
Astronomy and Astrophysics
Astronomy
Scientific paper
We explore the transmission spectrum of the Neptune-class exoplanet GJ 436b , including the possibility that its atmospheric opacity is dominated by a variety of non-equilibrium chemical products. We also validate our transmission spectrum code by performing tests for model atmospheres that use purely analytic Rayleigh scattering and water vapor opacities. For GJ 436b, the relative coolness of the planet's atmosphere, along with its implied high metallicity, may make it dissimilar in character compared to "hot Jupiters." Some recent observational and modeling efforts suggest low relative abundances of H2O and CH4 present in GJ 436b's atmosphere, compared to calculations from equilibrium chemistry. We include these characteristics in our models and examine the effects of absorption from methane-derived higher order hydrocarbons. To our knowledge, the effects of these non-equilibrium chemical products on the spectra of close-in giant planets have not previously been investigated. Significant absorption from HCN and C2H2 are found throughout the infrared, while C2H4 and C2H6 are less easily seen. We perform detailed simulations of JWST observations, including all likely noise sources, and find that we will be able to constrain chemical abundance regimes from this planet's transmission spectrum. For instance, the width of the features at 1.5, 3.3, and 7 microns indicates the amount of HCN versus C2H2 present. The NIRSpec prism mode will be useful due to its large spectral range and the relatively large number of photo-electrons recorded per spectral resolution element. However, extremely bright host stars like GJ 436 may be better observed with higher spectroscopic resolution mode in order to avoid detector saturation. We find that observations with the MIRI low resolution spectrograph should also have high signal-to-noise in the 5 - 10 microns range due to the brightness of the star and the relatively low spectral resolution (R 100) of this mode.
Fortney Jonathan J.
Freedman Richard Stuart
Greene Thomas P.
Shabram Megan
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